Inside of a beam tetrode tube with anode cut open. The beam plates are the silver coloured structures to the left and rightTwin beam tetrode RCA-815, perhaps most famously used as the bias oscillator tube in the Ampex Model 300 "bathtub" 1/4" full-track professional audio tape recorder

A beam tetrode, sometimes called a "beam power tube", is a type of tetrodevacuum tube (or 'valve') with auxiliary beam-focusing plates designed to augment power-handling capability and help reduce unwanted emission effects. These tubes are usually used for power amplification, especially at audio-frequency.[1]

A tetrode is a vacuum tube having four active electrodes. The four electrodes in order from the centre are: a thermionic cathode, first and second grids and a plate. There are several varieties of tetrodes, the most common being the screen-grid tube and the beam tetrode. In screen-grid tubes and beam tetrodes, the first grid is the control grid and the second grid is the screen grid. In other tetrodes one of the grids is a control grid, while the other may have a variety of functions.

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.

Contents

History

The problem of secondary emission from the anode (US: plate) in the tetrode tube was solved by Philips/Mullard with the introduction of a suppressor grid which resulted in the pentode design. Since Philips held a patent on this design, other manufacturers were keen to produce pentode type tubes without infringing the patent. In the UK, two EMI engineers, Cabot Bull and Sidney Rodda, produced and patented an alternate design in 1932.[2] Their design had the following features (compared to the normal pentode).

Secondary emission in physics is a phenomenon where primary incident particles of sufficient energy, when hitting a surface or passing through some material, induce the emission of secondary particles. The term often refers to the emission of electrons when charged particles like electrons or ions in a vacuum tube strike a metal surface; these are called secondary electrons. In this case, the number of secondary electrons emitted per incident particle is called secondary emission yield. If the secondary particles are ions, the effect is termed secondary ion emission. Secondary electron emission is used in photomultiplier tubes and image intensifier tubes to amplify the small number of photoelectrons produced by photoemission, making the tube more sensitive. It also occurs as an undesirable side effect in electronic vacuum tubes when electrons from the cathode strike the anode, and can cause parasitic oscillation.

Koninklijke Philips N.V. is a Dutch multinational technology company headquartered in Amsterdam, one of the largest electronics companies in the world, currently focused in the area of healthcare and lighting. It was founded in Eindhoven in 1891 by Gerard Philips and his father Frederik, with their first products being light bulbs. It was once one of the largest electronic conglomerates in the world and currently employs around 74,000 people across 100 countries. The company gained its royal honorary title in 1998 and dropped the "Electronics" in its name in 2013.

Mullard Limited was a British manufacturer of electronic components. The Mullard Radio Valve Co. Ltd. of Southfields, London, was founded in 1920 by Captain Stanley R. Mullard, who had previously designed thermionic valves for the Admiralty before becoming managing director of the Z Electric Lamp Co. The company soon moved to Hammersmith, London and then in 1923 to Balham, London. The head office in later years was Mullard House at 1–19 Torrington Place, Bloomsbury, now part of University College London.

The control and screen grids were wound so that the pitches were the same and the wires were in alignment (the pentode used different pitches).

A pair of beam-forming plates was added at the two ends of the oval grid structure to focus the electron stream into a pair of beams 180 degrees apart (the pentode added a third grid). These plates are normally connected to the cathode.

The control grid is an electrode used in amplifying thermionic valves such as the triode, tetrode and pentode, used to control the flow of electrons from the cathode to the anode (plate) electrode. The control grid usually consists of a cylindrical screen or helix of fine wire surrounding the cathode, and is surrounded in turn by the anode. The control grid was invented by Lee De Forest, who in 1906 added a grid to the Fleming valve to create the first amplifying vacuum tube, the Audion (triode).

The design is today known as the beam tetrode but historically was also known as a kinkless tetrode, since it is a four-electrode device without the negative resistance kink in the anode current vs anode voltage characteristic curves of a true tetrode. Some authorities, notably outside the United Kingdom, argue that the beam plates constitute a fifth electrode.[3][4]

In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.

The EMI design had the following advantages over the pentode:

The screen grid current was about 5-10% of the anode current compared with about 20% for the pentode, thus the beam tetrode was more power-efficient.

The beam tetrode had smaller transconductance than a similar pentode and therefore required a larger input signal at the control grid for a given output power.

The beam tetrode had more tendency to oscillate than a pentode if the circuit was not designed and laid out properly.

A push–pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load. Push–pull outputs are present in TTL and CMOS digital logic circuits and in some types of amplifiers, and are usually realized as a complementary pair of transistors, one dissipating or sinking current from the load to ground or a negative power supply, and the other supplying or sourcing current to the load from a positive power supply.

A transformer is a static electrical device that transfers electrical energy between two or more circuits. A varying current in one coil of the transformer produces a varying magnetic flux, which, in turn, induces a varying electromotive force across a second coil wound around the same core. Electrical energy can be transferred between the two coils, without a metallic connection between the two circuits. Faraday's law of induction discovered in 1831 described the induced voltage effect in any coil due to changing magnetic flux encircled by the coil.

Negative feedback occurs when some function of the output of a system, process, or mechanism is fed back in a manner that tends to reduce the fluctuations in the output, whether caused by changes in the input or by other disturbances.

The MOV (Marconi-Osram Valve) company, under the joint ownership of EMI and GEC, considered the design too difficult to manufacture due to the need for good alignment of the grid wires. As MOV had a design-share agreement with RCA of America, the design was passed to that company. RCA had the resources to produce a workable design resulting in the 6L6. Not long after, the beam tetrode appeared in a variety of offerings, including the 6V6 in December 1936, the MOV KT66 in 1937 and the KT88 in 1956, designed specifically for audio and highly prized by collectors today.

Some tubes described as pentodes are actually beam tetrodes. The ubiquitous Mullard EL34 (6CA7), although manufactured by Mullard as a pentode, was also produced by other manufacturers as a beam tetrode instead.

A beam tetrode family widely used in the US comprised the 25L6, 35L6, and 50L6, and their miniature versions the 50B5 and 50C5. This family is not to be confused with the 6L6 despite similar designations. They were used in millions of All American Five AM radio receivers. Most of these used a transformerless power supply circuit. In American radio receivers with transformer power supplies, built from about 1940-1950, the 6V6, 6V6G, 6V6GT and miniature 6AQ5 beam tetrodes were very commonly used.

In military equipment, the 807, and 1625 with rated anode dissipations of 25 watts and operating from a supply of up to 750 volts, were in widespread use as the final amplifier in radio-frequency transmitters of up to 50 watts output power and in push-pull applications for audio. These tubes were very similar to a 6L6 but had a somewhat higher anode dissipation rating and the anode was connected to the top cap instead of a pin at the base. Large numbers entered the market after World War II and were used widely by radio amateurs in the USA and Europe through the 1950s and 1960s.

The beam tetrode produces the lowest distortion of this class of tube by producing significantly less third-harmonic distortion, and lower intermodulation distortion when used in ultralinear mode. Even-harmonic distortion is automatically cancelled in a push-pull design. The beam tetrode can be operated as a triode (by connecting its screen grid to its anode), and in this mode functions more efficiently than a pentode operated in the same manner.[6]

Related Research Articles

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s. Valve amplifiers are used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, audiophile stereo amplifiers, military applications and very high power radio and UHF television transmitters.

The pentagrid converter is a type of radio receiving valve with five grids used as the frequency mixer stage of a superheterodyne radio receiver.

KT66 is the designator for a beam tetrode vacuum tube introduced by Marconi-Osram Valve Co. Ltd. (M-OV) of Britain in 1937.

A Williamson amplifier is a type of vacuum tube (valve) amplifier whose circuit design uses the same principles as a design published by D.T.N. Williamson. Williamson proposed the standard which became generally accepted as the target figure for high-quality audio power amplifiers, for less than 0.1% total harmonic distortion at full rated power output.

6L6 is the designator for a vacuum tube introduced by Radio Corporation of America in July 1936. At the time Philips had already developed and patented power pentode designs, which were rapidly replacing power triodes due to their greater efficiency. The beam tetrode design of the 6L6 allowed RCA to circumvent Philips' pentode patent.

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube, which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode consists of an evacuated glass envelope containing five electrodes in this order: a cathode heated by a filament, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.

The EL34 is a thermionic valve or vacuum tube of the power pentode type. It has an international octal base and is found mainly in the final output stages of audio amplification circuits and was designed to be suitable as a series regulator by virtue of its high permissible voltage between heater and cathode and other parameters. The American RETMA tube designation number for this tube is 6CA7. The USSR analog was 6P27S.

A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted. Single-ended amplifiers normally operate in Class A; push-pull amplifiers can also operate in Classes AB or B without excessive net distortion, due to cancellation.

A valve audio amplifier (UK) or vacuum tube audio amplifier is a valve amplifier used for sound reinforcement, sound recording and reproduction.

The 6AQ5 is a miniature 7-pin (B7G) audio power output beam tetrode vacuum tube with ratings virtually identical to the 6V6 at 250 V. It was commonly used as an output audio amplifier in tube TVs and radios. There are versions of this tube with extended ratings for industrial application which are designated as 6AQ5A, and 6AQ5W/6005 or 6005W.

The KT88 is a beam tetrode/kinkless tetrode vacuum tube for audio amplification.

Ultra-linear electronic circuits are those used to couple a tetrode or pentode vacuum-tube to a load.

A valve RF amplifier or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical radio frequency signal.

Technical specifications and detailed information on the valve audio amplifier, including its development history.

Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.